PROJECT SUMMARY: This project aims to illuminate the mechanisms and structures of viral enzymes that cap the 5' end of messenger RNA. The m7GpppN cap is a distinctive feature of eukaryal cellular and viral mRNA that is required for mRNA stability and translation. Capping entails three enzymatic reactions: (i) the 5' triphosphate of the pre-mRNA is hydrolyzed to a diphosphate by RNA triphosphatase (TPase); (ii) the diphosphate RNA end is capped with GMP by RNA guanylyltransferase (GTase); and (iii) the GpppN cap is methylated by RNA (guanine-N7) methyltransferase (MTase). Whereas the three capping reactions are universal in eukarya and DNA viruses, there is a remarkable diversity in the genetic organization of the cap- forming enzymes in different taxa and in different viruses, as well as a complete divergence in the structure and catalytic mechanism of the TPase enzymes found in fungi, protozoa, and several large DNA viruses versus the human TPase. These differences can be exploited to develop novel anti-infective agents directed against capping of the pathogen's mRNAs. They also provide instructive clues to eukaryal phylogeny and evolution of large DNA viruses. This proposal focuses on the poxvirus mRNA capping enzyme, a heterodimer composed of TPase, GTase and MTase domains fused within a single large polypeptide subunit, plus a smaller subunit that binds and stimulates the MTase domain. We plan to functionally map the active sites and domain interfaces of the poxvirus capping enzyme, guided by a new crystal structure of the complete capping enzyme heterodimer. As an outgrowth of our studies of RNA end-modification by viral enzymes, we've developed a new line of research into end-healing and end-sealing enzymes that repair programmed RNA breaks. Programmed RNA damage is an ancient mechanism of responding to cellular stress and distinguishing self from non-self. RNA damage also figures prominently in host responses to virus infection. tRNA damage inflicted by a latent anticodon nuclease PrrC (which breaks tRNALys) underlies a potent host innate immune response to bacteriophage T4 infection, which is thwarted by a virus-encoded tRNA repair system consisting of T4 Pnkp (polynucleotide kinase/phosphatase) and T4 Rnl1 (RNA ligase 1). We have dissected the mechanism, structure, and specificity of Pnkp and Rnl1, and we have extended our analysis to discover or characterize new RNA repair enzymes from viruses, bacteria and human cells. RNA damage and repair now comprise a significant component of the research effort supported by this grant. The current plan focuses on two aspects of the host-pathogen RNA damage/repair dynamic: (i) genetic and structural analysis of PrrC anticodon nucleases and (ii) characterization of a newly discovered two-component RNA repair system (Hen1-Pnkp) that is distributed widely among bacteria.